This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Nonheme iron enzymes catalyze a broad range of metabolically important transformations by activating dioxygen. In the proposed mechanisms of those enzymes, dioxygen is reduced one electron at a time to afford in turn iron(III)-superoxo, iron(III)-peroxo or hydroperoxo, and iron(IV)-oxo intermediates. An important strategy to gain insight into these mechanisms is by trapping and characterizing such intermediates either during enzyme turnover or in functional models of such enzymes. X-ray absorption spectroscopy has played a key role in establishing the structures of these trapped intermediates. In the following SSRL two-year period, we will study MMO intermediates P and Q in the presence of CD4, as well as high-valent iron-oxo intermediates of interest, including the first iron(V)-oxo species and new diiron(III,IV) and diiron(IV) intermediates, some of which will be generated by electrochemical oxidation. Studies on DAOCS, the 2-oxoglutarate-dependent enzyme that converts penicillin to cephalosporin, will also be initiated to address a mechanistic question posed by an unexpected crystallographic result.